The present invention relates to a method for chlorinating phthalic anhydride. More specifically, it relates to a method of photochlorinating phthalic anhydride vapor.
Several methods have been employed for direct chlorination of phthalic anhydride using chlorine. U.S. Pat. No. 4,297,283 reports that both phthalic anhydride and orthophthalonitrile may be chlorinated in the vapor phase using chlorine gas at temperatures above 350.degree. C., and usually below 600.degree. C. The optimum temperature is said to be between 350.degree. and 475.degree. C. The 4-chloro isomer is a favored product and the molar ratio of the 4-chloro to the 3-chloro compound ranges between 5 and 11. It is preferred to use an inert gas carrier such as nitrogen or helium, etc. in the reaction chamber. It is also preferred to use a molar excess of chlorine over the aromatic compound of between 0.5 to 20 moles of chlorine per mole of aromatic compound.
German patent DE 3,911,951, discloses a process for preparing chlorophthalates by chlorinating the parent compounds, in the gas phase, at a temperature range in from 300.degree. to 700.degree. C. The chlorine to substrate ratio is between 0.7:1 to 5:1 and the process employs activated carbon catalyst prepared by doping activated charcoal, zeolite, alumina, or silica with a metal salt or oxide from the first, second or eighth sub groups. In the chlorination of phthalic anhydride, the best example presented reaction product consisting of 70% 4-chlorophthalic anhydride, 1% 3-chlorophthalic anhydride, 3% dichlorophthalic anhydride, and 23% unreacted phthalic anhydride.
Japanese patent 62185082, as abstracted in CA 108:95092s, discloses the gas phase chlorination of phthalic anhydride in the presence of a catalyst consisting of iron compounds and activated carbon to produce tetrachlorophthalic anhydride. The process is conducted at a temperature of approximately 320.degree. C.
Japanese patent 60161974, as abstracted in CA 104:89172u, discloses the gas phase reaction of phthalic anhydride with chlorine at a temperature range of 200.degree.-400.degree. C. to form tetrachlorophthalic anhydride in the presence of a catalyst composed of CoCl.sub.2 and a component selected from CaCl.sub.2, BaCl.sub.2, and LaCl.sub.3, all deposited on active carbon.
The photochlorination of aromatic compounds is not always predictable. For example, U.S. Pat. No. 2,499,120 discloses that a mixture of benzene and chlorine may be photochlorinated to produce predominately the gamma isomer of hexachlorocyclohexane.
U.S. Pat. No. 4,268,457 discloses that photochlorination of paraphenoxytoluene produces chlorination on the side chain rather than on the ring. Accordingly, the product of the chlorination is paraphenoxybenzotrichloride.
U.S. Pat. No. 4,046,656 discloses that bromine acts as a catalyst in the photochlorination of methyl-substituted aromatic compounds. In this photochlorination process, all chlorines substitute on the methyl group, while the ring hydrogens are considered inert substituents, that is, they are not readily displaced by chlorine.
U.S. Pat. No. 4,643,811 discloses a photochemical process for the preparation of 1,1-dichloro-2,2,2-trifluoroethoxybenzene derivatives. The process involves photochlorination of 2,2,2-trifluoroethoxybenzene derivatives. The chlorination occurs at the .alpha.-carbon of the ethoxy side chain, rather than on the ring.
Zweig and Epstein (J. Org. Chem. vol. 43, p. 3690, 1978 (at page 3691)) report that as part of another research effort, they attempted to induce the liquid phase chlorination of phthalic anhydride using ultraviolet radiation. They report that the reaction could not be induced by ultraviolet irradiation at temperatures up to 235.degree. C.